The present invention relates to an electronic device for charging an accumulator from an electrical energy source. The invention is more particularly useful in an electronic wristwatch.
Such devices are known, particularly from U.S. Pat. No. 3,802,178 which describes a device for charging an accumulator by a photo-electric cell or any other electric energy source through a diode. The diode is used to prevent the accumulator from discharging into the photo-electric cell if the cell cannot produce a voltage sufficient to charge the accumulator, for example in the event of insufficient illumination.
The disadvantage of this device is that the energy source has to overcome a voltage of approximately 0.5 volt (threshold voltage of the diode). As a result, a substantial amount of energy is lost. Such a waste should be avoided in the case of an electronic watch, as it considerably reduces the battery lifetime.
U.S. Pat. No. 3,731,474 also discloses a device limiting the voltage produced by a solar cell charging an accumulator. This device comprises a bipolar NPN transistor, the emitter and collector of which are respectively connected to the positive and negative poles of the solar cell, and the control electrode to the positive pole of the accumulator. A diode is inserted between the positive poles of the solar cell and the accumulator. When the illumination of the cell becomes strong, the current circulating in the diode produces a voltage drop both across the terminals thereof and at the input of the transistor. Such a drop is sufficient to render the transistor conductive and thus shunt a non negligible amount of current towards the negative pole of the cell.
The disadvantage of this device is that a large amount of energy is lost in the transistor as well as in the diode.
German Patent application (DOS) No. 25 34 455 also describes a device for charging an accumulator from a battery. A transistor is connected between the battery and the accumulator. An electronic circuit controls the conduction of the transistor, which is directly proportional to the voltage drop between the battery and the accumulator. This conduction is adjusted so as to be just sufficient for the consumption of an electronic circuit connected to the terminals of the accumulator.
The disadvantage of this device is that the energy delivered by the battery is not fully utilised for charging the accumulator, because this energy is partly lost in the transistor for limiting the conduction thereof. Moreover, if the voltage drop between the battery and the accumulator is less than a given threshold (which is not negligible), the transistor remains blocked.